A power adapter aims to transform input power to output power to supply an electronic device. A wide variety of electronic devices on the market require electric power of differential potentials. In order to make the power adapter to supply power to various types of electronic devices, the general approach is like the one shown in FIG. 1 in which a power adapter 100 is equipped with a horizontal switch 101 and a potential scale 102 indicating potential values. Users can move the horizontal switch 101 to adjust output power potential according to the electronic device connecting to the power adapter 100 to meet requirement thereof. The horizontal switch 101 switches the potential by sliding. Such a movement often cannot position at an accurate potential, but rather between two potential values of the potential scale 102. As a result, an inaccurate output potential is delivered from the power adapter 100.
To solve the aforesaid problem, R.O.C. patent No. M336623 discloses a dual-input power supply with adjustable output voltage, in which including a case with a window formed thereon, a circuit board held in the case, a voltage regulation knob electrically connected to the circuit board and a rotary wheel with voltage adjustment numerals formed thereon visible through the window. The voltage regulation knob is coupled with a rotary stem of the rotary wheel. To switch the output power potential, turn the rotary wheel and the rotary stem and drive the voltage regulation knob turning on the circuit board. Through the voltage adjustment numerals on the rotary wheel accurate output power of the power adapter can be set and delivered.
While the aforesaid conventional techniques can switch the potentials of output power, the switch is located on the case of the power adapter and connected to the circuit board in the power supply. The circuit board has to be made at a larger size to accommodate wiring and the switch. As a result, the size of the power adapter is larger and the production cost also is higher as well. The larger size also makes carrying difficult and takes too much space.